Field of Technology
The present disclosure relates to methods for producing titanium and titanium alloy articles. In particular, certain non-limiting aspects of the present disclosure relate to methods including producing a hydrogenated titanium or titanium alloy, deforming (working) the titanium or titanium alloy, and subsequently dehydrogenating the material to reduce the hydrogen content of an article. In certain non-limiting embodiments of the method of the present disclosure, the method provides a titanium or titanium alloy article having an ultra-fine α-phase particle size, e.g., an average α-phase particle size of less than 10 microns in the longest dimension.
Description of the Background of the Technology
Titanium alloys are used in a variety of applications for their advantageous balance of material properties including strength, ductility, modulus, and temperature capability. For example, Ti-6Al-4V alloy (also denoted “Ti-6-4 alloy”, having a composition specified in UNS R56400) is a commercial alloy that is widely used in the aerospace and biomedical industries.
Titanium has two allotropic forms: a “high temperature” beta (“β”)-phase, which has a body centered cubic (“bcc”) crystal structure; and a “low temperature” alpha (“α”)-phase, which has a hexagonal close packed (“hcp”) crystal structure. The temperature at which the α-phase transforms completely into the β-phase as a titanium alloy is heated is known as the β-transus temperature (or simply “β-transus” or “Tβ”). Conventional processing of cast ingots of titanium alloys to form billets or other mill products generally involves a combination of deformation steps above and below the β-transus depending on the desired structure and material property requirements for a given application.
A finer α particle size can result in higher tensile properties, improved fatigue strength, and improved ultrasonic inspectability for the titanium alloy article. The conventional approach to achieving a finer α particle size in titanium alloy articles usually involves managing complicated thermo-mechanical processing, for example, rapid quenching from the β-phase field followed by relatively large amounts of hot working or strain in the α+β phase region and possibly a post-deformation anneal in the α+β phase region to enhance particle refinement. In particular, to achieve the finest α particle size, hot working at very low, and perhaps marginally practical, temperatures and using relatively low, controlled strain rates is required. However, there are manufacturing limits to what can be achieved with this conventional approach, due to increased forging loads, lower process yields due to cracking, and lack of or limits of practical strain rate control, especially at large section sizes. The conventional approach may also be limited by an increasing tendency to form small voids or pores in the alloy under certain processing conditions such as low temperatures and/or high strain rates. This phenomenon is known as “strain-induced porosity” or “SIP.” The presence of SIP in the alloy can be particularly deleterious to the alloy properties and can result in significant process yield loss. In severe cases, additional and costly processing steps, such as hot-isostatic pressing, may be required to eliminate SIP that has formed. Thus, there has developed a need for methods for producing titanium alloy articles having a finer α particle size while avoiding limits imposed by the hot working temperature and/or the strain rate.